As a conventional actuator, there has been provided an actuator, as shown in FIG. 7, in which a plane extending in parallel to the length of an output axis is formed in such a way as to construct a rotation prevention mechanism. In a case in which such an actuator is connected to an external device, a joint 11 connected to the output axis of the actuator is connected to an end of a link plate 13 with a pin 20 and a locking ring 21 in such a way that the joint 11 can rotate around a central axis GG, and another end of the line plate 13 is connected to a lever 2 of the external device with a pin 14 and a locking ring 15 in such a way that the link plate 13 can rotate around a central axis HH, as shown in FIG. 8. Patent reference 1 discloses an actuator in which a notch extending along an axis is formed in an output axis, and a projection fitted into the above-mentioned notch is formed in a casing of the actuator in such a way as to construct a rotation prevention mechanism.    [Patent reference 1] JP, 2002-327709, A
A problem with the conventional actuators constructed as mentioned above is that in a case in which a rotation prevention mechanism is disposed in the output axis, the outer diameter of the output axis has to be made thick in order to ensure the mechanical strength of the output axis, and therefore the actuator becomes enlarged. Another problem is that the provision of the rotation prevention mechanism increases the manufacturing cost of the actuator. A further problem is that when connected to an external device, because the locking rings are used in the following two places a connecting place at which the joint is connected to the link plate, and a connecting place at which the link plate is connected to the lever, the distance c from the link plate 13 to the locking ring 21, i.e., the play of the connecting portion in the axis direction becomes large. A still further problem is that there is a play due to the rotation prevention mechanism also in the actuator, and sticking occurs depending on the positional relation between the actuator and the external device when the actuator is attached to the external device.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide an actuator which can prevent sticking from occurring and can reduce its manufacturing cost, which can be downsized, and which is equipped with a connecting member for connecting the actuator to an external device.